The Riekse 2Fe-2S electron transfer (ET) proteins are ubiquitous in nature and differ significantly from other 2Fe-2S ET proteins. One such difference is the ability of Riekse proteins to couple proton and electron transfer processes. This property appears to be due to the unusual imidazole/imidazolate coordination of one iron in the 2Fe-2S cluster. The strategy of this proposal is to utilize model complexes of the Riekse 2Fe-2S center as an efficient way to determine how controlled variation of imidazole-type ligands changes the redox potential, stability, geometry and proton transfer properties of the 2Fe-2S cluster. The use of model complexes allows modifications to the 2Fe-2S center that would be impossible with the protein and facilitates the isolation and spectroscopic characterization of each redox active state. The spectroscopic properties, primarily NMR, Mossbauer and EPR, of the model complexes will be compared to those observed from the Riekse proteins. This investigation will clarify how biological ET at redox active metal centers is modulated by coordination properties. Such structure-function relationships are key to our understanding of biochemical processes. Additionally, determining substrate affinity for Rieske model complexes will aid our understanding of how antibiotics which bind to the Rieske protein inhibit ET.